Your 86 seems a better deal, if some questions are asked. Does the seller know if the head gaskets have been replaced? Most all of the 6.9s need it around 100,000 miles, as the factory gaskets give out between cyls.#6&8 (rear two on the pass. side).
Cavitation is not generally a problem with the 6.9, no worries there.
Have you driven it? The front diff sometimes has a "death wobble" at driving speed (around 50 mph, usually) that takes some work and $ to correct. Does it have speed control? Does it work? Does it have the stock water separator? What gear ratio? 4.10 or 3.55? Or do you care about fuel mileage?
Has it been run on anything but diesel (WMO, biodiesel, etc.)? If it has ever been run on untaxed fuel, it will have a red dye color in the tank and system that could be problems if the law stops you for anything else.
These and some other questions could help you negotiate a better price, but $2500 is not too bad if the body is straight.